In the description that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art.
Magnetic susceptibility is the degree of magnetization of a material in response to a magnetic field. The dimensionless magnetic susceptibility is represented by the Greek symbol X. Materials having a positive magnetic susceptibility are paramagnetic. Paramagnetism is the tendency of the atomic magnetic dipoles in a material that is otherwise non-magnetic to align under the influence of an external magnetic field. This alignment of the atomic dipoles with the magnetic field tends to strengthen it.
Materials having a negative magnetic susceptibility are diamagnetic. Diamagnetism is a weak form of magnetism that is only exhibited in the presence of an external magnetic field. It is the result of changes in the orbital motion of electrons due to the external magnetic field. The induced magnetic moment is small and in a direction opposite to that of the applied field. When placed between the poles of a strong electromagnet, diamagnetic materials are attracted towards regions where the magnetic field is weak. Superconductors are perfect diamagnets. When a superconducting material is placed in an external magnetic field it will expel the field lines from its interior. Superconductors also have zero electrical resistance, which is a consequence of their diamagnetism.
Ferromagnetism is a phenomenon by which a material can exhibit a spontaneous magnetization, and is one of the strongest forms of magnetism. The magnetic susceptibility of a ferromagnetic substance is not linear. The response is dependent upon the state of the sample and can occur in directions other than that of the applied field. The differential susceptibility Xij=dMj/dHi expresses the susceptibility as a function of the derivatives of the components of the magnetization with respect to components of the applied field. The magnetization (M) of the material (the magnetic dipole moment per unit mass) and the applied field (H) are typically measured in Amperes per meter.
Magnetic property measurements can be used to characterize materials such as paramagnetic materials, diamagnetic materials, ferromagnetic materials, and superconducting materials. Typically, magnetic properties can be measured using either alternating electrical current (AC) or direct electrical current (DC) magnetic fields. AC susceptometers, for example, can detect the variation in flux created by placing a sample into an alternating magnetic field, and can measure as a function of frequency and/or temperature the complex susceptibility (X′+iX″), which comprises both a real component (X′) and an imaginary component (X″). The imaginary component is related to the energy losses (e.g., energy absorbed) by the sample from the AC field.